xfs: create a separate hashname function for extended attributes

xfs: create a separate hashname function for extended attributes

Source kernel commit: a64e0134754bf88021e937aa34f1fbb5b524e585

Create a separate function to compute name hashvalues for extended
attributes.  When we get to parent pointers we'll be altering the rules
so that metadump obfuscation doesn't turn heinous.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>